1. Field of the Invention
This invention relates to an image forming apparatus utilizing an electrophotographic method, such as a copying machine, a printer, a facsimile or the like.
2. Description of the Related Art
In conventional color image forming apparatuses such as copying machines, printers or the like, an intermediate transfer method is used where toner images formed in a plurality of image forming sections are sequentially superimposed on an intermediate transfer body and then transferred onto a recording medium altogether.
An example of an embodiment of an image forming apparatus utilizing this intermediate transfer method will be explained using FIG. 6. First, electrostatic latent images are formed on a plurality of photographic photosensitive drums 101a, 101b, 101c, and 101d by using light, magnet, electric charge or the like, and then respective toners having respective colors are provided from respective developers 102a, 102b, 102c, 102d to form toner images, thereby developing (visualizing) the electrostatic latent images.
An intermediate transfer belt 103 as an intermediate transfer body is arranged to be suspended by a drive roller 105, a driven roller 106 and a tension roller 107 so as to be disposed opposite each of the photosensitive drums 101a, 101b, 101c, and 101d along a straight, horizontal plane, and the toner images formed on the surfaces of the photosensitive drums 101a, 101b, 101c, and 101d are sequentially transferred onto the intermediate transfer belt 103 in a superimposed manner by the action of primary transfer blades 104a, 104b, 104c and 104d, as primary transfer means, disposed on an opposite side of the intermediate transfer belt 103 from the respective photosensitive drums 101a, 101b, 101c, and 101d. 
A secondary transfer roller 108 is disposed on a side of the intermediate transfer belt 103 opposite the driven roller 106, as a secondary transfer means, and sheets P composed of papers, synthetic resins or the like serving as recording media are suitably fed from sheet cassettes 109a, 109b, or a manual feed tray 110 to a position between the intermediate transfer belt 103 and the secondary transfer roller 108 where the toner images transferred on the intermediate transfer belt 103 are re-transferred onto the sheets P by the action of the secondary transfer roller 108.
The sheets P on which the toner images have been transferred are subjected to permanent fixation through heating and pressurizing treatments by a fixing device 111, including upper and lower fixing rollers 111a, 111b, and then delivered out of the apparatus. Using the photosensitive drums 101a, 101b, 101c and 101d as image forming means of Yellow, Magenta, Cyan and Black in a sequential order can give full color images; or using any one of the photosensitive drum 101a, 101b, 101c, or 101d can give a single color image (e.g., black).
The image forming apparatus with the above-mentioned intermediate transfer method is structured so that toner images transferred from the photosensitive drums 101a, 101b, 101c and 101d as intermediate transfer bodies onto the intermediate transfer belt 103 are read (detected) using an image reading means 112, and that based on image information detected/read, image quality improvement is promoted.
The image reading means 112, for instance, reads the image density of a toner image formed on the intermediate transfer belt 103, and then, based on the information thus obtained, image forming conditions or primary transfer conditions are changed; or relative output image positions of respective photosensitive drums 101a, 101b, 101c and 101d are detected based on the positions of the respective color toner images formed on the intermediate transfer belt 103, and then, in order to correct the amount of slippage with respect to the position accuracy, the image forming timing or the image forming positions of the photosensitive drums 101a, 101b, 101c and 101d are thereby corrected.
In the aforementioned conventional example, however, the fixing device 111 where the toner image on the sheet P is heated, melted and fixed on the sheet P heats up its surrounding environment because the fixing device 111 is required to generate quite a huge amount of heat to perform its function. Heat thus generated from the fixing device 111 thermally affects various parts arranged around the fixing device, and especially in the case where the heat reaches the intermediate transfer belt 103, thermal expansion of the belt is generated in the vicinity of the fixing device 111, thermal contraction occurs as the belt moves away from the fixing device 111, and rotation of the intermediate transfer belt 103 makes this phenomenon repeating.
Accompanied by the heat stretching and contraction of the intermediate transfer belt 103 as mentioned above, toner images used for detection on the intermediate transfer belt 103 (detection toner images) become stretched and contracted, and especially in the case where toner images with a plurality of colors are superimposed on the intermediate transfer belt 103a, color slippage problem in terms of position arises.
Also, the heat capacity of the intermediate transfer belt 103 becomes smaller because of its structural function, so the appearance of influences by the aforementioned heat stretching and contraction are easily observed, and consequently there is posed a problem of lowered image quality, such as color slippage or the like.
In addition, a problem arises where, when the heat of the intermediate transfer belt 103 affects the image reading means 112, this causes optical parts such as optical lenses constituting the image reading means 112 to generate thermal expansion or distortion, thereby lowering the image reading accuracy.
Furthermore, when detection toner images are detected on a peripheral surface of the intermediate transfer belt 103 winding around a roller which supports and suspends the intermediate transfer belt 103, the fixing heat causes a change in the roller diameter, resulting in a lowering of the detecting accuracy.
The same problems as mentioned above occur in image forming apparatuses utilizing conveyance belts for carrying and conveying recording materials in order to sequentially transfer toner images from respective photosensitive drums onto the recording materials.
It is an object of the present invention to provide an image forming apparatus capable of improving a detecting accuracy of a detecting means which detects toner images formed on a movable body.
Other objects of the present invention will be apparent by reading the following detailed description of the preferred embodiments.